The present invention relates to a spectrophotometer and, more particularly, to a spectrophotometer using a plurality of sensors, dispersing elements having different optical characteristics, or a combination thereof.
The spectrophotometer has a spectroscope for dispersing a light from a light source and taking out a monochromatic light and branches the monochromatic light from the spectroscope into two light fluxes by a branch mirror. One of the two light fluxes is led to a standard side optical path and the other is led to a sample side optical path. The branched monochromatic lights pass through the standard side optical path and the sample side optical path which are held in an air state in which a substance to be measured is not yet set, respectively, and enter the sensors and are photoelectrically converted by the sensors. An electric signal corresponding to an amount of light which has passed through the standard side optical path is R. An electric signal corresponding to an amount of light which has passed through the sample side optical path is S. A ratio (S/R) between the electric signals R and S is taken out as a result of the measurement. A correction amount such as to always keep the ratio (S/R) constant is calculated. The calculated correction value is stored into a memory in a computer. After that, when a sample is actually set on the sample side optical path and measured, the correction value stored in the memory is read out and the measured value is corrected by the correction value.
The above method is a conventional base line correcting method, in which measurements are performed in the air state with respect to both of the standard side and the sample side and the difference between the results of the measurements is corrected as a difference when a sample is actually measured.
In the case of executing measurements for a whole wavelength range to be measured by using one sensor and one dispersing element, no problem occurs when using the above base line correcting method. However, in the case where a highly accurate measurement is required for a wide wavelength range or the like, it is necessary to use a plurality of sensors or dispersing elements which bear measurements of different wavelength ranges. However, sensitivity characteristics and spectrum characteristics of the sensors or dispersing elements which bear the different wavelength ranges are different. Particularly, the sensitivity characteristics or dispersing characteristics of two sensors or dispersing elements remarkably deteriorate in the overlap wavelength regions between adjacent wavelength ranges. Therefore, an unreproducible difference easily occurs in the measured values in such overlap wavelength regions.
As apparatuses regarding such a kind of apparatus, there have been disclosed apparatuses in, for instance, JP-U-62-39299, Japanese Patent Serial No. 968560, and the like.
According to the above conventional base line correcting method, a difference between R and S is detected and the difference amount is merely corrected upon actual measurement of a sample in a manner such that the ratio (S/R) of the energy S which passes through the sample side optical path to the energy R which passes through the standard side optical path becomes constant. Consequently, consideration lacks with respect to the following points.
1. No consideration is paid to a point that the sensitivity characteristics of two kinds of sensors are very low in the overlap wavelength regions and a point that the detection signals suddenly decrease in a very weak direction.
2. No consideration is paid to a point that although measurements to determine the correction amount are executed in a manner of air-to-air with regard to both of the standard side and the sample side, in the case of actually setting a sample and measuring the sample, situations in which the light after passing through the sample enters the photosensitive surfaces of the sensors are different due to influences by a refractive index, a reflectance, and a deflecting characteristic of the sample depending on the shape and material of the sample.
In both of the above two cases (1) and (2), the results of the measurements which were obtained in the case where both of the standard side optical path and the sample side optical path were held in the air state are used as references. Thus, there is a problem such that a difference occurs between the result obtained by the actual measurement of the sample and the correction amount.